Document our compatibility promise

[pellared]

This is inspired by how the Go standard library handles such issues. See: https://pkg.go.dev/time#Time

I would say we do our best to follow https://go.dev/doc/go1compat which is kind of:

what compiled in the past should compile in the future

with some exceptions.

The Collector does have it documented here: https://github.com/open-telemetry/opentelemetry-collector/blob/main/VERSIONING.md#go-modules

Probably we should also document it in our https://github.com/open-telemetry/opentelemetry-go/blob/main/VERSIONING.md

Originally posted by @pellared in https://github.com/open-telemetry/opentelemetry-go/pull/6272#discussion_r1943782946